1. Field of the Invention:
This invention relates to a method of manufacturing amorphous silicon compounds.
2. Description of the Prior Art:
W. E. Spear et al. discovered in 1976 that the conductivity of amorphous silicon obtained by the plasma decomposition of silane (SiH.sub.4) could be notably improved by substitutional doping with PH.sub.3 or B.sub.2 H.sub.6. D. E. Carlson et al. manufactured a trial solar cell using amorphous silicon in 1976. These achievements have drawn attention to amorphous silicon, and have given an impetus to researches devoted to improvement of the conversion efficiency of a thin-film solar cell using amorphous silicon.
The researches so far have led to the conclusion that an appropriate thin-film amorphous silicon photovoltaic cell is of the Schottky barrier type, p-i-n type, MIS type, or heterojunction type. Particularly, the first three types promise to provide highly efficient solar cells. The Schottky barrier type photovoltaic cell made by D. E. Carlson et al. in 1977 showed a conversion efficiency of 5.5%, the MIS type photovoltaic cell made by J. I. B. Wilson et al. in 1978 showed a conversion efficiency of 4.8%, and the pin type photovoltaic cell made by Yoshihiro Hamakawa in 1979 showed a conversion efficiency of 4.5%.
In a thin-film amorphous silicon solar cell, a carrier has a short life time in a p or n type doped layer, and a relatively long life time in an intrinsic amorphous silicon layer. It is, therefore, considered that in order to increase the efficiency of a thin-film amorphous silicon solar cell, it is important to improve the properties of its intrinsic amorphous silicon layer.
J. C. Knights and K. Tanaka examined the relationship between the magnitude of radio-frequency power as required for the plasma decomposition of silane, and the physical properties of intrinsic amorphous silicon thereby obtained. They both reported the advisability of employing a lower radio-frequency power. See Supplement to Japanese Journal of Applied Physics, Vol. 18-1, 101 (1979), and Journal of Non-Crystalline Solids, Vols. 35 and 36, 475 (1980), respectively.
The inventors of this invention have conducted extensive research and experiments for improving the conversion efficiency of a thin-film amorphous silicon solar cell. As a result, they have discovered that the optimum radio-frequency power for improving the conversion efficiency of such a solar cell is not always low as reported by J. C. Knights and K. Tanaka, but depends on the distance L between the radio-frequency electrode (cathode) for glow discharge and the solar cell substrate. This discovery has led to the conclusion that the conversion efficiency of a solar cell can be maximized if the distance L between the radio-frequency electrode and the cell substrate is appropriately selected, and that amorphous silicon of improved performance can be obtained if the substrate on which amorphous silicon is deposited is positioned in the vicinity of the end of a positive column formed by glow discharge.